Typically, a digital imager array includes a focal plane array of pixel cells, each one of the cells including a photo sensor, e.g., a photo gate, photoconductor, or a photodiode. In a CMOS imager a readout circuit is connected to each pixel cell which typically includes a source follower output transistor. The photo sensor converts photons to electrons which are typically transferred to a storage node, e.g., a floating diffusion region connected to the gate of the source follower output transistor. A charge transfer device (e.g., transistor) can be included for transferring charge from the photo sensor to the floating diffusion region. In addition, such imager cells typically have a transistor for resetting the storage node to a predetermined charge level prior to charge transference. The output of the source follower transistor is gated as a pixel output signal by a row select transistor.
Exemplary CMOS imaging circuits, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. Nos. 6,140,630, 6,376,868, 6,310,366, 6,326,652, 6,204,524, and 6,333,205, each assigned to Micron Technology, Inc. The disclosures of each of the forgoing patents are hereby incorporated by reference in their entirety.
With reference to FIGS. 1 and 2, which respectively illustrate a top-down and a cross-sectional view of a conventional CMOS pixel cell 100, when incident light 187 strikes the surface of a photodiode photo sensor 120, electron/hole pairs are generated in the p-n junction of the photodiode photo sensor 120 (represented at the boundary of n-accumulation region 122 and p+ surface layer 123). The generated electrons (photo-charges) are collected in the n-type accumulation region 122 of the photo sensor 120. The photo-charges move from the initial charge accumulation region 122 to a floating diffusion region 110 via a transfer transistor 106. The charge at the floating diffusion region 110 is typically converted to a pixel output voltage by a source follower transistor 108 and subsequently output on a column output line 111 via a row select transistor 109.
Conventional CMOS imager designs, such as that shown in FIG. 1 for pixel cell 100, provide approximately a fifty percent fill factor, meaning only half of the cell 100 is dedicated to converting light to charge carriers. As shown, only a small portion of the cell 100 comprises a photo sensor 120 (e.g., photodiode). The remainder of the pixel cell 100 includes isolation regions 102, shown as STI regions in a substrate 101, the floating diffusion region 110 coupled to a transfer gate 106′ of the transfer transistor 106, and source/drain regions 115 for reset 107, source follower 108, and row select 109 transistors having respective gates 107′, 108′, and 109′. In the conventional pixel cell 100 layout, each of the reset, source follower and row select transistor gates 107′, 108′ and 109′ are aligned consecutively, sharing source/drain regions 115 and requiring significant surface area for each pixel cell 100. As desired scaling of pixels continues to decrease the pixel's 100 total area, it becomes increasingly important to create high sensitivity photo sensors that utilize a minimum amount of surface area, or to find more efficient pixel cell layouts to minimize the area required by non-photosensitive components of the pixel cell in order to maintain relatively large areas for the photo sensors.
In addition, conventional storage nodes, such as floating diffusion region 110, have a limited amount of charge storage capacity. Once this capacity is reached, a pixel cell becomes less efficient. Once the charge storage capacity is exceeded, an undesirable phenomenon, known as “blooming” occurs, whereby the “over-capacity” charges escape to undesirable parts of the pixel cell 100 or to adjacent pixel cells. One suggested solution for dealing with this limited charge storage capacity is to add a capacitor which is connected to the floating diffusion region 110. The capacitor is used to store the additional over-capacity charges so the charges do not flow to other areas of the cell or adjacent cells. The problem with this solution, however, is that the additional capacitor takes up space in the cell that could otherwise be used to increase the size of the cell's photo sensor, thereby decreasing the potential fill factor for the pixel cells and overall array.
Accordingly, there is a need and desire for a more efficient pixel cell array architecture that has an improved fill factor and charge storage capacity.